Josephson Effect and Energy Gap Measurements from Nb/YBCO Point Contact Structures

Abstract

Besides its intrinsic interest, weak coupling between superconductors is widely recognized as a powerful probe to infer important informations on the underlying physics of superconductivity in general¹⁾. Accordingly many authors involved in the study of the new classes of high T_c superconductors have focused their attention on careful investigations of various types of superconductive junction structures ^4–7). The difficulty of preparing YBCO materials in films (though at IBM in particular there were successful attempts ⁸) has suggested to begin with point contact configuration rather than with sandwich tunneling barrier type junction. However, even point contacts appear to be rather complicated. The main problem stems from the strong normal-conducting surface layer present on the YBCO material. This circumstance implies that in order to establish a real link the contact pressure has to be very high producing very likely drastic damages to the sample. In the present work we have realized point contact structures very reliable and stable, made by a Nb point pressed on a bulk high-T superconductor (Y Ba₂Cu₃O₇) by using a suitable apparatus. Current-voltage characteristics and dV/dl curves have been obtained showing both the occurrence of Josephson effect and providing an estimate of the gap of the YBCO.